1. Field of the Invention
This invention relates to a method of and apparatus for reading a radiation image where an image signal is obtained by integrating by integrating amplifier charge signals output from a radiation image sensor which stores a radiation image upon exposure to radiation bearing thereon the radiation image and outputs charge signals according to the stored radiation image upon exposure to reading light.
2. Description of the Related Art
Various radiation image sensors on which a radiation image of an object is recorded upon exposure to radiation passing through the object and which outputs charge signals representing the recorded radiation image have been proposed and put into practice.
As the radiation image sensor, there has been known those employing semiconductor material which generates electric charges upon exposure to radiation. As such radiation image sensors, there have been proposed those of so-called an optical-reading system.
As the radiation image sensor of the optical-reading system, a radiation image sensor comprising a first electrode layer permeable to radiation, a recording photoconductive layer which generates electric charges upon exposure to radiation, a charge transfer layer which behaves like a substantially insulating material to the electric charge in the same polarity as a latent image and behaves like a substantially conductive material to the electric charge in the polarity opposite to that of the latent image, a reading photoconductive layer which generates electric charges upon exposure to reading light, and a second electrode layer in which a plurality of linear electrodes permeable to the reading light are arranged in parallel to each other, the layers being superposed one on another in this order, is proposed in U.S. Pat. No. 6,770,901.
In the radiation image sensor, radiation is projected onto the radiation image sensor from the first electrode layer side and electric charges generated in the recording photoconductive layer are stored on the interface between the recording photoconductive layer and the charge transfer layer, thereby recording a radiation image. When a linear reading light beam scans the radiation image sensor in the longitudinal direction of the linear electrodes, electric charges are generated in the reading photoconductive layer and the electric charges are combined with the stored electric charges and flow into the linear electrodes. The charge signals flowing into the linear electrodes are integrated by integrating amplifiers (so-called charge amplifiers) connected to the linear electrodes, whereby an image signal is obtained and the radiation image is read out.
FIG. 9 shows a timing chart representing the relation between the reading light beam projecting time and the integrating time of the integrating amplifiers when the radiation image is read out as well as the amplitude of the signal current output from the linear electrodes of the radiation image sensor and the amplitude of the output voltage of the integrating amplifiers upon exposure to the reading light beam. It is assumed that a radiation image has been recorded on the radiation image sensor in the distribution shown in FIG. 9, that is, linear images extending in the same direction as the reading light source are recorded in parallel at predetermined intervals in the scanning direction of the reading light source, i.e., in the direction of arrow X in FIG. 9.
As shown in FIG. 9, conventionally the recording light beam is projected onto the radiation image sensor by continuously moving the reading light source in the longitudinal direction of the linear electrodes, and the image signal components (making up the image signal) are obtained for each integrating time by turning on and off the reset switches for the integrating amplifiers in response to the movement of the reading light source.
However, projection of the reading light beam is disadvantageous in that since the radiation image sensor is slow in response speed to the reading light, for example, the signal current output from the radiation image sensor in response to projection of the reading light beam for integrating time S1 can be continuously output from the radiation image sensor even in the next integrating time S2 as shown in FIG. 9 and accordingly, an output voltage is output from the integrating amplifier as an image signal component also in the integrating time S2 when the signal current is to be 0 (the hatched portion in FIG. 9), which forms noise and deteriorates the image quality such as the sharpness of the read radiation image.
Further, it has been proposed to use as the reading light source a pulsed light source in place of the light source which continuously emits the reading light, that is, to intermittently project the recording light beam for integrating times. However, even in this system, it is preferred that the reading light projecting time be as long as possible and a method of ensuring a sufficient reading light projecting time and at the same time avoiding generation of the noise due to delay in response of the radiation image sensor to the reading light has not been found.
There has been known so-called correlation double sampling as a method of obtaining an image signal by the use of an integrating amplifier. In the correlation double sampling, an image signal is obtained on the basis of the difference between the output of the integrating amplifier just after the initiation of the integration and just before the termination of the integration. Accordingly, if the reading light is projected onto the radiation image sensor upon sampling of the output of the integrating amplifier just after the initiation of the integration, the signal current output from the radiation image sensor in response to projection of the reading light creates an offset voltage and deteriorates the S/N of the read radiation image.